An electro-active polymer (EAP) is a soft actuator that can generate high stress and large strain. A dielectric elastomer actuator (DEA) is an EAP which uses a dielectric elastomer film or membrane with a flexible electrode attached to each face. Application of a voltage difference across the electrodes generates Maxwell stress. The two electrodes are urged together by electrostatic attraction of free charges on the electrodes, causing the thickness of the membrane to decrease and the surface area of the faces to increase. This transformation is further effected by electrostatic repulsion of like charges on each individual face. Thus, application of a voltage to the electrodes causes the membrane to expand in area and compress in thickness.
Pelrine et al. in “Electrostriction of polymer dielectrics with compliant electrodes as a means of actuation”, Sensors and Actuators A 64, 77-85 (1998) describe the basic principles behind DEAs and a linear actuator based thereon.
DEAs are described by Masaki Haruna et al. in “Development of Soft Actuator: Mechanism with Vibration Element Using Dielectric Elastomer to Generate Large Displacement”, Electroactive Polymer Actuators and Devices, Proc. of SPIE Vol. 6524, 652418 (2007). Masaki et al. also describe a vibration element that acts as a linear actuator and has a dielectric elastomer held at its edges by opposing frame members. The element includes two pairs of opposing electrodes formed on the elastomer. The first pair is positioned at a first end of the element and the second pair is positioned at the second opposing end and spaced apart from the first pair, such that the first pair of electrodes is isolated from the second pair. Each pair forms a driven area and they are actuated alternately through application of a voltage to each pair of electrodes. A push-rod is attached to the elastomer (preferably at the boundary between the driven areas) and moves backwards and forwards as the voltage is applied to the alternate pairs of electrodes. A ratchet mechanism enables the push-rod to drive an output-rod in a single direction.
Zhou, J., Huang, M., Wang, X. and Song, W. in “Development of a magnetostrictive drive rotary motor driven by circular parallel movement”, Smart Materials and Structures 16 (2007) 2063-2066 describe a rotary motor constructed of three magnetostrictive actuators connected to a stator frame with a rotor placed in the centre of the stator. Magnetostriction is the change in shape of a material due to a change in its magnetization. Movement of the actuators is scaled up using bulky flexible flexure hinges and combined using a mechanism which enables a pure rotation movement for driving the rotor of the motor.
U.S. Pat. No. 6,084,321 describes a rotary actuator having a substantially planar sheet of anisotropic conducting polymer, portions of which may be activated in sequence to produce crank rotation. U.S. Pat. No. 6,806,621 describes arrangements using EAPs to rotate a crank.